Cellular phones are a ubiquitous sight in today's societies. Presently there is an interest in enhancing the functionality and user experience in using portable electronic apparatus such as cellular telephones. To that end various steps toward making portable electronic devices multimedia capable, such as the introduction of color screens, adding electronic cameras, and enhancing wireless network bandwidth, have been undertaken.
One area in which efforts have been made to improve the user's experience, is audio quality and tactile stimulation. Tactile stimulation is related to audio in the sense that low frequency tactile stimulation can be used as a surrogate for low frequency audio, which would be difficult to generate using a small size limited power system that can be built into a portable device.
Both audio, and tactile stimulation can be provided by a single device known as a multi-function transducer (MFT). Certain types of MFT comprise a resiliently mounted speaker cone connected to a voice coil, and a resiliently mounted magnetic assembly that provides a magnetic field in which the voice coil operates. The resiliently mounted magnetic assembly and the speaker cone can be driven to oscillate by applying signals to the voice coil. The magnetic assembly owing to its mass and the compliance of its mounting will oscillate at a relatively low frequency within the range of frequencies that are easily perceptible by tactile sensation. Alternatively, a separate vibrating device for providing tactile stimulation, and a separate speaker for generating audio are used.
Whether MFTs or separate audio and tactile transducers are used, for large size portable devices, in order to produce sufficient tactile force, it may be desirable to include multiple transducers. When multiple MFTs are used together, in spite of the fact that both may be receiving the same single frequency signal, an undesirable amplitude modulation of the response of the two MFTs has been observed.
Additionally, even when using a single MFT, or dedicated tactile vibration transducer, owing to manufacturing variances, the resonant frequency may vary to an extent that the performance variations, when using a predetermined frequency drive signal, may reach unacceptable levels.
In using a vibrating device to alert a user to some event, such as the receipt of a wireless communication message, an issue to be contended with is the limited power handling capacity of small vibrating devices that are suitably sized for inclusion in portable devices. In certain circumstances, such as when a user is in place with loud music or using an apparatus that also generates vibration, the stimulus generated by a small tactile alert may, in effect, be drowned out by other stimulus. Thus, it is important to maximize the stimulus that can be created by a small, relatively low power tactile alert.